function getScrollTop() {
    var  scrollTop = 0, bodyScrollTop= 0,  documentScrollTop= 0;
    if (document.body) {
        bodyScrollTop=document.body.scrollTop;
    } ;
if (document.documentElement) {
        documentScrollTop=document.documentElement.scrollTop;
    }
    scrollTop= (bodyScrollTop - documentScrollTop > 0) ?bodyScrollTop :documentScrollTop;
    return scrollTop;
}
//文档的总高度
function getScrollHeight() {
        var scrollHeight = 0,  bodyScrollHeight= 0,  documentScrollHeight= 0;
        if(document.body)   bodyScrollHeight=document.body.scrollHeight;
    if (document.documentElement) {
        documentScrollHeight=document.documentElement.scrollHeight;
    }
    scrollHeight= (bodyScrollHeight - documentScrollHeight > 0) ?bodyScrollHeight:documentScrollHeight;
    return scrollHeight;
}
//浏览器视口的高度
function getWindowHeight() {
    var windowHeight = 0;
    if(document.compatMode == "CSS1Compat") {
        windowHeight=document.documentElement.clientHeight;
    } else{
        windowHeight=document.body.clientHeight;
    }
    return windowHeight;
}


//滚动事件触发
window.onscroll = function() {
    if(getScrollTop() + getWindowHeight() == getScrollHeight()) {
        //触发下拉事件
        var onDropdownEvent = new CustomEvent('onDropdown', {});
        if(window.dispatchEvent) {
            window.dispatchEvent(onDropdownEvent);
        } else {
            window.fireEvent(onDropdownEvent);
        }
    }
}